1. Field of the Invention.
This invention relates to hydrocyclones and, more particularly, to an air-sparged hydrocyclone apparatus and method.
2. The Prior Art of Classification.
The term "size reduction" is applied to all the ways in which particles of solids are cut or broken into smaller pieces. Comminution is a generic term for size reduction and there are various types of comminuting equipment available. The objective of the comminuting equipment is to produce small particles from larger ones, the smaller particles being desired either because of their large surface area or because of their shape, size, number, etc. Reducing the particle size has the advantage in that it increases the reactivity of solids; permits separation of unwanted ingredients by mechanical methods; and reduces the bulk of fibrous materials for easier handling. Throughout the process industries, solids are reduced by different methods for different purposes. For example, chunks of crude ore are crushed to workable size; synthetic chemicals are ground into powder; sheets of plastic are cut into tiny particles so that the geometric characteristics of particles, both alone and in mixtures, are important in evaluating the product from the comminuting equipment. Addtionally, commercial products must often meet stringent specifications regarding the size and sometimes the shape of the particles they contain.
During size reduction, the particles of feed material are first distorted and strained. The work necessary to strain the particles is stored temporarily in the solid as mechanical energy of stress, just as mechanical energy can be stored in a coil spring. As additional force is applied to the stressed particles they are distorted beyond their ultimate strength until they suddenly rupture into fragments, generating new surface. The ratio of surface area created by crushing to the energy absorbed by the solid is a measure of the crushing efficiency. The energy efficiency of the comminution operation may be thus measured by the new surface created upon reduction in size. Unlike an ideal system, actual comminution equipment does not yield a uniform product, whether the feed is uniformly sized or not. The product always consists of a mixture of particles, ranging in size from a definite maximum to a submicroscopic minimum. Some machines, especially grinding devices, are designed to control the magnitude of the largest particles in their products with little control over the fine sizes. In other types of grinding devices the production of fine sizes is minimized although not entirely eliminated.
The operating and capital costs associated with size reduction are the highest of all the unit operation costs encountered in the mineral processing industry and the cost of energy is a major portion of the operating cost. The relative magnitude of the unit operation costs in mineral processing plants are as follows: crushing, 15%; grinding, 45%; concentration, 25%; solid/liquid separation, 5%; material transport, 5%; and miscellaneous, 5%. Of these costs, the most significant is the cost incurred in operation of the grinding circuit, particularly with regard to the amount of energy consumed. It is estimated that greater than one percent of our nation's energy consumption is used for size reduction processes. As a consequence, closed-circuit grinding systems are one of the most important unit operations in the mineral processing industry and a great deal of attention has been directed toward improving the efficiency of this particular operation. Very frequently, the economic success of an entire plant will be limited by its ability to grind material to the required size specification at the desired rate.
Closed-circuit grinding is understood to involve size reduction (typically a tumbling mill, or the like) and size separation (typically a classifier). The coarse particles from the size separation are recycled to the size reduction equipment, hence the term "closed-circuit grinding." Basically two types of closed-circuit grinding operations are employed. In the first type, the fresh feed initially passes to the size reduction device (tumbling mill) followed by size separation (classification) and recycle of the coarse particles to the fresh feed. In the second type of closed-circuit grinding, fresh feed enters the size separator first with the coarse product passing to size reduction and after size reduction, rejoining the fresh feed for further classification.
Generally, these circuits are operated to maximize the production of a product with certain size specifications. It is well-documented in the literature that increased capacity can be achieved by operating at circulating loads of 200 percent or greater so that operating plants generally follow this practice. Another approach to enhance grinding circuit capacity is to grind at a higher percent solids in the mill, thus increasing throughput at no increase in power consumption. Finally, many engineers are attempting to optimize and control the performance of closed grinding circuits in order to increase capacity. Each of these techniques has resulted in varied success for improved grinding circuit capacity while relatively little attention has been focused on the classification technique and the efficiency of size separation as it is currently practiced in the industry.
However, one of the most important factors in determining the capacity of a closed grinding circuit is the efficiency of size separation. Size separation (classification) is typically accomplished with mechanical classifiers or hydrocyclones, the latter being preferred in the design of new plants. It is intuitively evident that if misplaced fine material of the desired size range is being returned along with coarse material to size reduction, the mill capacity will be reduced correspondingly. Under these circumstances, the mill will be regrinding material which is already of a suitable size. If, on the other hand, the fine material is not misplaced in the coarse material stream, the mill will have a greater capacity and the fresh feed rate can be increased.
The effect of classifier efficiency on the grinding circuit capacity is revealed in at least two computer simulation studies. In one analysis, examination of the results reveals that the grinding circuit capacity could be increased by as much as 50 percent by improved classifier efficiency. The results from another simulation suggests the grinding circuit capacity could be increased by as much as 64 percent if perfect size separation could be achieved. In view of the fact that the efficiency (as measured by the coefficient of separation which represents the fraction of feed material separated ideally) of most hydrocyclones, even under the best of circumstances, is only 50 percent and that the efficiency of mechanical classifiers is even lower, considerable improvement in grinding circuit capacity could be achieved by improved classifier efficiency.
Many excellent publications describe the operation of the hydrocyclone which is a cylindricoconical piece of equipment into which a suspension of particles is pumped under moderate pressure (10 psig, for example). The suspension is fed tangentially through a feed port causing rotation of the suspension. The flow of the suspension consists of a downward-spinning, outer spiral close to the cyclone wall and an upward-spinning, inner spiral along the axis of the hydrocyclone when oriented in a vertical direction. Particles in the suspension settle radially in the centrifugal field and those with greater mass are carried downwardly by the outer spiral and are discharged through the apex opening of the cone.
The major portion of the liquid and fine particles (coarse particles together with residual fines having been removed in the outer spiral) are forced to leave the cyclone through the overflow nozzle or vortex finder in the upward-spinning, inner spiral along the axis of the cyclone. Inside the inner spiral, a low pressure is generated creating a vortex which collects all of the air that has been carried in as bubbles or dissolved in the feed water. This visible air core is focused and stabilized by the vortex finder which extends a prescribed distance into the cylindrical section of the hydrocyclone. Because of the increase in circumferential speed of the inner spiral, higher centrifugal forces are generated which assist in keeping large particles from entering the inner spiral of the suspension so that ideally, these large particles would be prevented from reporting to the fine product collected in the overflow.
It is evident that the characteristics of the slurry fed to the cyclone influence the cut point or separation size. The particle size distribution in the slurry determines the relationship between the relative amounts of coarse product and fine product obtained. The effective slurry viscosity also influences the separation size and is determined by the solids content in the feed. Higher slurry concentrations therefore generate coarser cuts than lower concentrations. This effect can also be described in terms of hindered settling, because the movement of the coarser particles is hindered by the zone of smaller particles, through which the coarser ones must pass. The viscosity of the liquid itself acts in the same way. Furthermore, the difference in specific gravity between different particles as well as the difference in specific gravity between particles and the liquid phase is important. The shape of the particles is also important. Very flat particles such as mice tend to go to the overflow even though they may be relatively coarse. Also, overflow and underflow size distributions may be influenced by other factors such as mechanical wear which may cause continual change in the cyclone performance. Predictions of performance based on calculations from first principles are, therefore, most difficult.
To restate the nature of the flow in the hydrocyclone, particles in the suspension experience a centrifugal force which causes them to move at some radial velocity, depending upon their mass and the other factors set forth hereinbefore, toward the wall of the hydrocyclone. This radial "settling" velocity of the particles is opposed by the radial flow of the liquid toward the axis; so that, ideally, the particles will be distributed radially according to their mass. The relative magnitude of these velocity terms will determine the radial position of a given size and density particle. Between the upwardly spinning, inner spiral and the downwardly spinning, outer spiral there exists a surface of zero axial (longitudinal or vertical) velocity. Those particles which lie inside the surface of zero axial velocity (the smaller particles) will be transported through the vortex finder to the overflow. The coarse particles will be positioned outside the surface of zero axial velocity, with some thrown against the cyclone wall, and consequently, these particles will be transported through the apex to the underflow. As a result of these considerations, a size separation occurs between particles of given specific gravities.
As mentioned previously, the efficiency of this separation is far from perfect and various attempts have been made to improve the quality of the size separation process. Of course, improved efficiency can be realized by doing a two-stage separation, a technique which is practiced in some instances. Also, multiple entry systems for the feed have been suggested in order to improve cyclone performance. Some investigators have designed hydrocyclones to allow for tangential water injection through ports in the conical section with improved efficiencies having been reported, evidently due to elutriation of fine particles from the underflow product. A hydrocyclone similar to these latter designs has been marketed by Krebs Engineers, Menlo Park, Calif., for a number of years but has not had great popularity in the mineral processing industry. It appears that water injection has at least two disadvantages which are; increased difficulty in balancing water flows for specified product pulp densities; and a limited amount of water injection in order to avoid destruction of the flow pattern in the hydrocyclone. Importantly, optimum functioning of a hydrocyclone depends on constant conditions in the feed, especially the volumetric flow rate. For example, it is believed important in the prior art that air must not be sucked into the system by the feed pump since such fluctuations would tend to destroy established flow patterns and alter the steady state condition.
Numerous publications dealing with mineral processing plants, grinding circuits, and the theory, application, and operation of hydrocyclones are available, some of which are listed below
1. A. L. Mular and R. B. Bhappu, Mineral Processing Plant Design, SME/AIME, p. 101 (1978).
2. A. B. Cummins and I. A. Given, editors, SME Mining Engineering Handbook, 2, p. 31--31 (1973).
3. L. G. Austin and P. T. Luckie, "Grinding Equations and the Bond Work Index," SME/AIME Trans. 252, p. 259 (1972).
4. J. A. Herbst, G. A. Grandy and D. W. Fuerstenau, "Population Balance Models for the Design of Continuous Grinding Mills," X International Mineral Processing Congress, Institution of Mining Metallurgy, London, paper 19, (1973).
5. D. A. Dahlstrom, "Fundamentals and Applications of the Liquid Cyclone," Chemical Engineering Prog. Symp. Ser. No. 15, 50 p. 41-61 (1954).
6. D. F. Kelsall, "A Study of the Motion of Solid Particles in a Hydraulic Cyclone," Trans. Institute of Chemical Engineering, 30, p. 87-108 (1952).
7. H. Travinski, "Theory, Applications and Practical Operation of Hydrocyclones," Eng. Min. J; p. 115-127, Sept. (1976).
8. D. Bradley, The Hydrocyclone, Pergamon Press, 330 pp. (1965).
9. A. J. Lynch, Developments in Mineral Processing, Mineral Crushing and Grinding, Elsevier, p. 87-120 (1977).
10. M. D. Brayshaw, "Use of a Numerical Model to Sharpen the Hydrocyclone Efficiency Curve," Ph.D. Thesis, Department Chemical Engineering, University of Natal, Durban South Africa (1978).
11. D. A. Dahlstrom, "High Efficiency Desliming by Use of Hydraulic Water Additions to the Liquid-Solid Cyclone," Mining Engineering and AIME Transactions, p. 188, August (1952).
12. D. F. Kelsall and J. A. Holmes, "Improvement of Classification Efficiency in Hydraulic Cyclones by Water Injection," V International Mineral Processing Congress, Institution of Mining and Metallurgy, London, p. 159 (1960).
In view of the foregoing, it would be an advancement in the art to provide a novel hydrocyclone apparatus and method for improving the separation of fine particles from coarse particles in the hydrocyclone. Another advancement in the art would be to provide an improved hydrocyclone apparatus and method wherein an air sparge is introduced into the hydrocyclone apparatus for assisting in separating the fine particles from the coarse particles so that more efficient removal of fine particles in the overflow can be achieved.
3. The Prior Art of Dense Media Cyclones.
The use of dense media cyclones is well-established in the art, particularly in the area of coal preparation. This separation is based on the difference in specific gravity between components of a particulate mixture rather than on the basis of size. The equipment and basic flow patterns are essentially the same as discussed in the previous section. Certain modifications are made to accentuate separation based on specific gravity rather than size, the most significant of which is a much larger cone angle for the hydrocyclones. To accomplish this separation, a fine dispersion of magnetite or ferrosilicon is intentionally added to the system to prepare an effective liquid phase, the specific gravity of which is between the specific gravities of the two components of the feed material. The feed component with the lower specific gravity is removed in the overflow while the feed component with the higher specific gravity is removed in the underflow. The dense media is recovered and recycled.
Conventional hydrocyclones are used in this fashion to separate coal from waste as well as other cyclonic devices marketed specifically as dense media cyclones, such as the Dyna Whirlpool. A useful discussion of some of the features of these commercial models may be found in the publication COAL PREPARATION, 3rd Edition, Leonard and Mitchell, editors, SME/AIME, New York, 1968.
It would, therefore, be a further advancement in the art to provide a novel air-sparged hydrocyclone and method for use in a dense media separation mode to promote separation based on the differences in specific gravity between the components in the slurry.
4. The Prior Art of Froth Flotation.
Froth flotation involves the aggregation of air bubbles and mineral particles in an aqueous media with subsequent levitation of the bubble-particle aggregates to the surface and transfer to the froth phase. Various publications are extant on this subject. Whether or not bubble attachment and aggregation occurs is determined by the degree to which the particle's surface is wetted by water. When the surface shows little affinity for water, the surface is said to be hydrophobic (water hating) and an air bubble will attach to the surface. Accordingly, separation is based on controlled differences in particle hydrophobicity. Any water present at a hydrophobic surface can be replaced by air due to the relative magnitudes of the surface energies comprising the system. As a result, a contact angle is established which provides a measure of the surface's hydrophobicity. Since water is a polar molecule, it will only hydrate or wet a polar surface and a hydrophobic surface reflects a lack of surface polarity.
The stability of the attachment of the air bubble is measured by the contact angle developed between the three phases. When the air bubble does not displace the aqueous phase, the contact angle is zero. On the other hand, complete displacement of the water represents a contact angle of 180 degrees. Values of contact angle between these two extremes provide an indication of the degree of surface hydration, or the hydrophobic character of the surface. There are no known solids that exhibit a contact angle greater than about 105 degrees which is the value obtained with paraffin. There are few naturally hydrophobic minerals (coal, molybdenite, sulfur, talc, pyrophyllite) all of which exhibit contact angles less than 105 degrees. Most minerals are hydrophilic and as such, must acquire their hydrophobic character by the adsorption of surfactants, termed collectors, in order to achieve selective froth flotation separations.
Few minerals are naturally hydrophobic. Most minerals on fracture and breakage expose polar surfaces which are readily wetted by water. These paticles can selectively be made hydrophobic by surface chemical reactions with flotation reagents. These reagents frequently contain polar and non-polar groups in order to effect the desired hydrophobicity.
Among the flotation reagents used are those which are generally termed collectors and frothers. A collector is a reagent which adsorbs at the solid-liquid interface in such a fashion as to present a hydrophobic surface. A frother is a reagent which adsorbs at the air-water interface, the resulting reduction in surface tension establishes in the froth phase and this reagent is frequently an alcohol derivative. Activators and depressants are also identified as flotation reagents, usually inorganic, and serve to modify the behavior of the system. For example, an activator enables adsorption of the collector and is in itself generally incapable of creating a hydrophobic surface. A depressant prohibits adsorption of the collector and thus aids in maintaining selectivity.
The conventional flotation cell is, in essence, a stirred-tank reactor with certain provisions for air injection, air dispersion mechanisms, and froth removal. Conventional froth flotation circuits include a rougher section, a scavenger section, and a cleaner section which can be identified in any set of flotation cells. The rougher section is designed to establish good recovery with only a small consideration given to the grade of the product obtained. A scavenger section is designed to pick up anything missed by the rougher section with even less consideration being given to grade. The cleaner section is designed to produce a product whose grade meets the desired specifications.
Among the common separations accomplished by froth flotation are included the separation of various sulfide ores such as lead-zinc ore and copper porphyry ore and separation of non-sulfide materials such as coal, iron ore, phosphate, and potash.
In these processes, the slow drainage of misplaced hydrophilic particles from the froth phase accounts, in large measure, for the inefficiency of the separation. Consequently, the separation is accomplished in multiple stages to enhance the quality of the separation. Even so, the standard flotation cell (stirred-tank reactor with provision for air dispersion) may be inadequate to make the desired quality of separation. As a result, these cells have been modified by various manufacturers in an attempt to achieve improved performance. In addition, other techniques have been suggested and tested such as column flotation.
Numerous publications are available in the art and two of the more recent books are cited below.
1. D. W. Furstenau, editor, Froth Flotation, 50th Anniversary Volume, SME/AIME, pp. 677 (1962); and
2. M. C. Furstenau, editor, Flotation, A.M. Gaudin Memorial, Volumes 1 and 2, SME/AIME, pp. 1341 (1976).
In view of these factors, it would be an even further advancement in the art to provide a novel air-sparged hydrocyclone by which hydrophobic particles could be separated from the hydrophilic particles of a suspension. Such a novel apparatus and method is disclosed and claimed herein.